The greatest single problem encountered in producing oil and gas wells is corrosion. Substantially all oil and gas wells produce water in varying amounts. They also produce either hydrogen sulfide or carbon dioxide, with the latter being the most common. These are soluble gases and when hydrogen sulfide is mixed with water it produces hydrosulfonic acid. When carbon dioxide is mixed with water it produces carbonic acid. Each of these acids is highly corrosive to ferrous metals.
Bottom hole temperatures in excess of 200.degree. F. are common in oil and gas wells and temperatures of that magnitude accelerate the corrosive effect of both acids. The severity of the corrosion problem is illustrated by those wells where the production tubing must be replaced at six month intervals.
When an oil well declines in production it usually ceases to flow naturally. When this occurs it is necessary to pump the oil from the production zone to the surface. The most efficient way to produce a large volume of fluid from the well is to use an electrically operated submergible pump unit which consists of an electric motor mounted on a pump which is attached to the down-hole end of the production tubing.
Electric powered oil well pumps frequently utilize high horsepower motors designed to operate on high voltage and low current. Accordingly, the power cable usually has conductors which are small in diameter and have thick insulation. Even if there is only slight degradation of the conductor insulation, a transient voltage "spike" in the electric power system may initiate corona. If, for example, a 4,160 volt pump motor is utilized, the operating voltage approximates the corona initiation voltage. Moreover, it takes less voltage to maintain corona than it does for it to be initiated. Therefore, once the corona is initiated, 4,160 volts may sustain it and this will cause rapid acceleration of the degradation of the insulation. The corona problem is further aggravated by the permeability of thermoplastics and elastomers used for insulation because both of these materials tend to degrade when exposed to fluids in a well bore.
The electric power cable which conducts electricity from a surface source to the motor is strapped to the outside of the tubing. When installed in this manner it is in the annulus between the interior of the casing and the exterior of the tubing. As previously mentioned, the annulus of a producing oil well is generally filled with completion fluid. In a well 10,000 feet deep--which is not uncommonly deep--the pressure exerted by the fluid column would be approximately 5,000 p.s.i. at the bottom of the annulus. The temperature at the bottom of the annulus in a well 10,000 feet deep would, of course, depend upon its location, but it would rarely be less than 200.degree. F. and often would be greater. Deeper wells have greater pressures at the bottom of the annulus and greater temperatures as well. As most shallow production has been discovered, the trend is to drill deeper wells with correspondingly adverse bottom hole conditions.
Substantially all conductor insulation used in down hole power cables is either an elastomers and thermosplastics--including those so-called impermeable elastomers and thermoplastics--are in fact permeable. Heat in excess of 200.degree. F. accelerates their degradation and their insulations are also particularly vulnerable to fluid under pressure because of their relatively low specific gravity.
In a high pressure wet environment such as that encountered at the bottom of an oil well, conductor insulation will fail because the pressure will force the fluids through the insulation into contact with the conductors and thus cause a short circuit. When bottom hole temperatures of 200.degree. F. or more are added to the above conditions the process accelerates.
If the conductor insulation fails in a down-hole pump, the tubing must be removed and reinstalled with new conductors. As previously stated, depending upon local conditions, either a drilling rig or a completion rig must be used to remove and reinstall the tubing. During this period the well is obviously not producing.
Wholly aside from the down hole conditions which adversely affect electric power cable, the process of installing the cable often results in its damage before it can be placed in service. As previously described, the cable is attached to the tubing which is inserted into the well bore. During the installation process the cable will be rubbed against the inside of the steel casing and may be abraded or actually cut. In this context, it must be recognize that lengths of casing are joined together by a casing collar which screws over the abutting ends. Generally, the two ends are not in contact with one another and this results in a gap in which the casing ends form a cutting edge when the insulation is forced against them.
Frequently the power cable is crushed between the tubing string and the inside of the casing. This occurs in the case of deviated offshore wells where the well bore extends laterally a distance of several thousand feet from the well site or production platform. In this situation the tubing may rest on the power cable at the bend section of the casing or grind the power cable against the bent section during installation.
Another frequent cause of power cable failure is deterioration of the cable insulation by the effect of electrical discharge which is referred to as "corona". Corona is defined as a "partial discharge" of electrical energy. Corona results from the dielectric breakdown of a portion of the path between two conductors with a voltage difference between the conductors. This discharge is always in the gaseous portion of the path between the conductors and is not present in the solid insulation material. When a voltage is applied to an electrical power system, portions of this voltage appear across each insulating segment of the power circuit. When the voltage across the air gap exceeds the dielectric breakdown strength of that particular thickness of air, breakdown of the air occurs, the air becomes conductive and this conductive nature of air is known as corona. The required voltage on the system for corona starting is dependent on a number of factors, including density of the air, the thickness of the air gap, the shape of the electrodes, the dielectric constant of the solid insulation and the thickness of the insulation. Moreover, where one of the conductors presents a sharp edge, projection, etc., the corona discharge will develop at a much lower starting voltage and will be maintained at a much lower operating voltage than if the electrodes present smooth surfaces across the air gap.
In order for corona discharge to develop, the voltage difference between the conductors must have a predetermined value, typically referred to as "starting voltage", in order for the air between the conductors to break down and become conductive. After the corona has developed, it is maintained at a much lower "maintenance voltage" between the conductors. When power circuits are operated at a voltage lower than starting voltage of the corona discharge, it would seem that corona discharge could not develop and the power system would thus be free from damage by the effect of corona. Even in such circuits, however, the voltage spikes and surges that occur in normal operation are typically sufficient for corona initiation. Once initiated, the corona continues until such time as the voltage difference across the conductors becomes insufficient for continuation. Since the insulating material for most conductors is typically a flexible elastomeric or thermoplastic, it is generally considered that the effects of corona cause deterioration and breakdown of the insulation causing cracks and deficiencies that may render the cable unserviceable. It is thus desirable to provide an electrical power cable that is free from corona discharge at all voltage levels. Where air is present within the cable, the cable shielding should be free from sharp internal edges and projections and should present an internal surface configuration which is not conducive to initiation and maintenance of corona discharge. In the alternative, or in addition to the provision of a specifically designed internal cable configuration, it may be desirable to eliminate any gaseous medium and thus effectively prevent the development of corona discharge.
In summary, as a well begins to decline in production it becomes corrosive and, when the decline reaches the point where it no longer flows to the surface naturally, it must be pumped. In the case of a corrosive well, the inhibitor must travel down the annulus to the bottom of the well. In the case of a submergible pump, the power cable must also travel down the annulus to the bottom of the well. In each case there is no conduit through which the inhibitor fluid or the electrical power may pass which can resist the abrasive, cutting and crushing forces of installation, and, in the case of electrical power cable, resist corona and the ambient conditions at the well bottom even in the event of a damage free installation.
In order to minimize the costly interruptions in production, as well as the deterioration of oil and gas producing equipment, it is the purpose of the present invention to provide a continuous, flexible, sealed, impervious metal chamber which affords protection against abrasive impact, crushing and tensile forces encountered in the bore hole of an oil or gas well. By means of this chamber, fluids and/or electricity may be transmitted to the bottom of the well by means of conductors which are isolated and protected from contact with the detrimental conditions existing within the bore hole.